1. Field of the Invention
The present invention relates generally to the control of an internal combustion engine powered by diesel fuel, and more specifically to reducing transient NOx generation produced by such a vehicle.
2. Background of the Invention
Controlling NOx emissions in diesel engines has posed significant challenges to the automotive industry. While emission control devices, such as NOx catalysts, can be used, these devices may be insufficient to meet ever-increasing emission standards.
A method to reduce NOx in diesel engines is the use of exhaust gas recirculation (EGR). EGR reduces NOx emissions during steady, or near steady, engine operation.
However, under transient engine operation in which a vehicle is required to accelerate, EGR can limit the performance of the vehicle by reducing the amount of airflow through the engine. EGR reduces airflow by displacing air in the combustion chamber, heating up the intake charge, and redirecting exhaust gas that would normally go through the turbocharger to the intake manifold. This last effect reduces the energy flow through the turbine, thus restricting the engine""s ability to create boost. This phenomenon can be dubbed xe2x80x9cthe EGR-Boostxe2x80x9d tradeoff.
Typically, conventional diesel systems suspend the use of EGR in order to accelerate aggressively. However, the inventors herein have recognized that without EGR, NOx emissions (concentration) increase dramatically. This comes at a time when the air mass flow rates are very high, causing NOx production to spike. Thus, the place where EGR is most needed is the place where it is not used.
The difficulty of this problem can be further appreciated by considering two types of EGR system that could be used with turbo-charged engines: the high pressure system and the low pressure system. The inventors herein have recognized the following disadvantages with each system.
In heavy duty applications, high pressure EGR systems have difficulty providing sufficient EGR flow at high load conditions, especially at low speed, since there is a negative pressure differential between the exhaust and intake manifold. One method to improve EGR flow is to throttle the engine. However, throttling the engine increases engine pumping work and decreases the gas flow to the engine.
Similarly, low pressure EGR systems allow only limited amounts of EGR under light load conditions, especially at low speed, since there is little pressure differential. Further, the low pressure EGR system adds significant purging volume that causes delays when trying to accelerate.
The above disadvantages are overcome by a system for an engine having an intake and exhaust manifold, the engine having a compression device coupled with a first portion coupled to the intake and a second portion coupled to the exhaust manifold of the engine, the system comprising: a first exhaust gas recirculation system having a first end coupled to the exhaust manifold upstream of the second portion of the compression device and a second end coupled to the intake manifold downstream of the first portion compression device, said first system also having a first valve that adjusts a first flow amount from the exhaust manifold to the intake manifold; a second exhaust gas recirculation system having a first end coupled downstream of the second portion of the compression device and a second end coupled to the intake manifold, said first system also having a second valve that adjusts a second flow amount from the exhaust to the intake manifold.
By providing multiple EGR loops, it is possible to reduce NOx emissions during high engine load, even in the presence of a compression device such as a supercharger. In other words, by using both a two EGR loops, one can obtain the benefits of each of the high pressure and low pressure EGR systems and thereby minimize the disadvantages of each system since the two systems complement each other.
Another advantage of the present invention is the ability to reduce transient NOx spikes.
Yet another advantage of the present invention is the ability of multiple loop EGR systems to enable the use of EGR throughout the entire engine map without using a throttle.